This invention relates to a method and apparatus for correcting defects in images, and in particular to a technique for correcting defects in images of documents, photographs, illustrations, and the like.
Images acquired by systems operating in the real world often contain defects or imperfections which cause the images to be inadequate as a starting point for subsequent use by people or machines. The defects or imperfections in images can arise from many causes, including imperfections in the optical system used to acquire the image, atmospheric effects, noise caused by electronic sensors, or other imperfections in the equipment or systems used to acquire the image. To compensate for these imperfections, researchers have long sought a wide range of methods for "restoring" the image.
The choice of the technique for image restoration in any particular application depends heavily upon the source of image degradation. One type of imperfection is common to images obtained from aerial cameras. Such images frequently exhibit blurring or distortion caused by camera motion or atmospheric effects. In these applications, the imaging device is physically distant from the object or objects whose image is being acquired.
Another class of image defects arises in document analysis. In document analysis, a part of the imaging device, for example the contact glass of a scanner, a photocopier, or the like, is physically close to the document. As a result, scratches and other blemishes are in virtually the same focal plane and introduce corresponding defects into the document image. Another imperfection in such systems arises from the electronic acquisition of data. Faulty electronic components can destroy the information content of the individual pixels, resulting in defects in the ultimate image produced by such equipment.
A well known technique for restoring information is to employ classical linear low pass filtering. In linear filtering, however, the diameter of the kernel employed for the filter must be at least on the same order as the diameter of the undefined pixel regions for useful computation to take place. This causes blurring to occur. Accordingly, low pass filtering has not been widely employed for image restoration in a document-handling context.
A further disadvantage of low (or high) pass filtering has been its tendency to alter valid image data, thereby degrading satisfactory portions of the image to enhance unsatisfactory portions. For an overall discussion of various digital image restoration techniques, see M. I. Sezan and A. M. Teklap, "Survey of Recent Developments in Digital Image Restoration," Optical Engineering (May 1990) 29(5):393-404.
Mathematical morphology methods have also been employed to recover information from noisy images using only local pixel information. See, for example, E. Doherty, An Introduction to Morphological Image Processing, SPIE Optical Engineering Press, Bellingham, Washington, 1992. Unfortunately, these methods rely on prior knowledge of the expected image, and so are not universally applicable.